Alien moons could be as habitable as exoplanets, astronomers say

Star Wars’ forest moon of Endor might be fiction, but astronomers say they’re hot on the trail of real-life alien moons -- which could also potentially be viable candidates for habitable worlds, researchers say.

Astronomers have found roughly 850 known alien worlds. And as announced at this week's American Astronomical Society meeting in Long Beach, the Kepler spacecraft has picked up 2,740 candidate planets since its 2009 launch. Scientists are looking for the slice of this population that lies in what’s known as the habitable zone, a region just close enough to the home star for liquid water to potentially exist.

Moons would be more challenging to find than planets – they’re much smaller, after all – but astronomers are on the hunt. A study to be published in January’s issue of the journal Astrobiology lays out the potential and drawbacks of habitable-zone moons.

“There’s got to be exomoons out there,” said study coauthor Rory Barnes, an astrobiologist at the University of Washington in Seattle. “There are a lot more moons in our solar system than there are planets, so it’s natural to expect that there’d be a lot more exomoons than exoplanets. It’s just a matter of finding them.”

There are two ways of picking out a moon, Barnes said. Kepler finds planets by staring into a patch of sky, waiting for planets to travel in front of their home star and block a little of its light. If the scientists pick up a smaller dip in the starlight either just in front of the planet's dip or just behind it, it could signal a circling moon.

Astronomers can also potentially find a moon by seeing if the planet crosses the star’s edge sooner or later than expected – signs of a wobble from a moon’s gravitational tug.

Moons are under very different forces than the planets they circle, Barnes pointed out.

For one thing, when a moon is close enough to its planet, it can't rotate freely – one side always faces the planet, because tidal forces from the planet lock it in place. Thus, the moon's ‘day’ isn’t caused by rotation on its own axis, but by making a full trip around the planet. This day could be short or long, because the more massive the planet is, the faster the moon travels. (The Earth’s moon takes 27.3 days to make a full circle around the small rocky planet, while Io, one of gas giant Jupiter’s moons, takes just 1.77 days.)

Tidal forces can also ‘squeeze’ a moon and heat it up past the point of habitability – which is why Io is a volcanic moon, Barnes said.

A moon’s environment could vary depending on your point of view, he explained. On the side looking outward into space, the moon would face its home star during daytime and the darkness of space during the night, as the Earth does.

The side of the moon facing the planet, however, could look very different. Its nights could be heated and illuminated by planet light, and its daytime would be punctuated by darkness as the planet briefly blocked out the home star.

Any life, if it existed, would have to adapt to such conditions.

“Life would certainly be different on an exomoon like that,” Barnes said.